Users Guide

1378 BGP
adds a discard route to RTO with prefix and network mask equal to those
defined for the aggregate address. Aggregate addresses apply to both locally-
originated routes and routes learned from peers.
Address aggregation is done prior to application of outbound policy. Thus, an
active aggregate may be advertised to a neighbor, even if the outbound policy
to the neighbor filters all of the aggregate's more specific routes (but permits
the aggregate itself).
An aggregate address is advertised with a set of path attributes derived from
the best paths for each NLRI included in the aggregate. Path attributes of the
aggregate are formed as follows:
• If one or more aggregated routes have ORIGIN set to INCOMPLETE, the
aggregate path sets ORIGIN to INCOMPLETE. Otherwise, if one or more
routes has an ORIGIN set to EGP, the aggregate path sets the ORIGIN to
EGP. Otherwise, the ORIGIN is set to IGP in the aggregate path.
• The local preference is set to the default local preference configured on the
router that creates the aggregate. (Of course, if the aggregate is advertised
to an external peer, the local preference is not included.)
• The NEXT_HOP is not imported from the aggregated routes. It is always
set to the local IPv4 address of the TCP connection to the peer.
• If the as-set option is configured for an aggregate, then the aggregate is
advertised with a non-empty AS_PATH. If the AS_PATH of all contained
routes is the same, then the AS_PATH of the aggregate is the AS_PATH of
the contained routes. Otherwise, if the contained routes have different
AS_PATHs, the AS_PATH attribute includes an AS_SET with each of the
AS numbers listed in the AS PATHs of the aggregated routes. If the as-set
option is not configured, the aggregate is advertised with an empty
AS_PATH.
• If BGP is configured to aggregate routes with different MEDs, no MED is
included in the path for the aggregate. Otherwise, if the as-set option is
not configured the aggregate MED is set to the MED for the aggregated
routes. If the as-set option is configured and the first segment in the AS
Path is an AS SET, then no MED is advertised.
• If the as-set option is configured, the aggregate's path does not include the
ATOMIC_AGGREGATE attribute. Otherwise, it does.
• The AGGREGATOR attribute is always included.